In an effort to improve the mechanical properties of existing materials used in applications where the performance of the material is critical, there is an on-going effort to form articles used in such critical applications from the cleanest possible materials. It has been determined that the presence of contaminants in metals used in critical applications is the source of scatter in mechanical properties, especially with respect to low-cycle fatigue properties. Therefore, the manner in which such materials are formed into the articles being used in such critical applications has been under scrutiny to eliminate, exclude, or minimize the detrimental effect of contaminants within the final article.
Where such articles are formed by powder metallurgy techniques, the powder production methods and materials handling practice is carefully controlled in order to prevent the introduction of undesirable contaminants. Where the article is formed by casting techniques, the handling of the molten material and the formation of that material into the desired end product must be carefully controlled in order to prevent the introduction of undesirable contaminants that will affect the properties of the final product. In some instances, it may also be possible to use thermomechanical processing to eliminate the detrimental effects of such contaminants when they are present.
The means by which these improvements are accomplished adds significantly to the cost of the article being produced and precludes their use in a number of applications. In addition, while some of the proposed methods of producing contaminant free metals are feasible on a laboratory scale, such processes are difficult to implement on an industrial scale.
Therefore, it is the principal object of the present invention to provide a method of casting metal which does not introduce contaminants into the casting and simultaneously removes from the material being cast the contaminants therein. It is a further object of the invention to provide a method for casting contaminant-free metal that can be readily adapted to industrial processes. It is a further object of the invention to provide a method for casting contaminant-free metal that can be utilized in a controlled atmosphere where access to the liquid metal dispensing container is restricted.